I'd better start doing my homework or I'll never get it done.
It's about time I started doing my homework. I've been putting it off forever.
1) I (had) better start doing my homework or I'll never get it done.
「そろそろ宿題しなきゃ終わらせられない。」
get it done 「終わらせる」
it's about time を使うなら
It's about time I started doing my homework. I've been putting it off forever.
「そろそろ宿題をする時間だ。ずっと延ばし延ばしにしてるよ。」
put it off 「延ばす」
2) I (had) better leave home soon otherwise I'll be late for school.
「そろそろ家をでないと学校に遅れる。」
it's about time を使うなら、
It's about time for me to leave the house. If I don't I'll be late for school.
「そろそろ家を出る時間だ。でないと学校に遅れる。」
ご参考になれば幸いです!